Refer to the exhibit. Which method must an architect use to provide connectivity between the mail servers?

ISATAP (Intra-Site Automatic Tunnel Addressing Protocol) is designed to connect individual IPv6 hosts within a single site over an IPv4 intranet, not to interconnect separate IPv6 networks.

6to4 uses the 2002::/16 prefix and embeds the router's public IPv4 address in bits 17-48 of the IPv6 address, enabling automatic encapsulation of IPv6 packets in IPv4 protocol 41 headers. This allows separate IPv6 networks - such as mail servers on different IPv6 segments - to communicate across an IPv4 infrastructure without manually configured tunnels between every pair of sites.

6rd (IPv6 Rapid Deployment) is a service-provider mechanism that uses ISP-assigned prefixes to deploy IPv6 over an ISP's IPv4 infrastructure, not a general-purpose site-to-site tunneling solution.

IPv4-compatible IPv6 addresses (::x.x.x.x) are deprecated per RFC 4291 and are not a supported or viable tunneling mechanism in modern networks.